Monte Carlo Effective Hamiltonian in 1+1 Dimensional Case'

Using a recently developed Monte Carlo effective Hamiltonian method,we study the low energy physics of 1+1 dimensional quantum mechanical system V(x)=μ²x²+λx⁴(here μ²<0,λ>0),which is similar to Higgs potential in the standard model of unified electroweak theory.Good results of the spectra,wavefunctions and thermodynamical observables are obtained.It shows that the new Monte Carlo Hamiltonian method has potential application to systems with many degrees of freedom and lattice gauge theory.     

Off-site repulsion-induced triplet superconductivity: a possibility for chiral p(x+y)-wave pairing in Sr2RuO4

In order to probe the effect of charge fluctuations on triplet pairing, we study the pairing symmetry in the one-band Hubbard model having the off-site Coulomb repulsion (V) on top of the on-site repulsion as a model for the gamma band of Sr2RuO4, a strong candidate for a triplet pairing superconductor. The result, obtained with the dynamical cluster approximation combined with the quantum Monte Carlo method, and confirmed from the fluctuation exchange approximation, shows that while d(x(2)-y(2)) pairing dominates over p in the absence of V, introduction of V makes p(x+y) and d(xy) dominant. The gap function for the chiral p(x+y)+ip(x-y) has nodes that are consistent with the recent measurement of specific heat in rotated magnetic fields in the ruthenate. This suggests that the off-site repulsion may play an essential role in triplet superconductivity in this material.     

Phase diagram of the ANNNI model in the Hamiltonian limit

The Hamiltonian limit of the ANNNI model in (1+1) dimensions is studied by using the Quantum Statistical Monte Carlo method. Even if recent results suggest that Monte Carlo calculations may prove unreliable in the study of this system, the phase diagram of the quantum version of the model was successfully obtained. In particular, the clusive transitions between the disordered, the floating incommensurate and the degenerate 2, 2 are determined by analysing the correlation length behaviour in finite lattices.Partially supported by CONICET Argentina     

Finite-temperature phase transitions in a two-dimensional boson Hubbard model

We study finite-temperature phase transitions in a two-dimensional boson Hubbard model with zero-point quantum fluctuations via Monte Carlo simulations of a quantum rotor model and construct the corresponding phase diagram. Compressibility shows a thermally activated gapped behavior in the insulating regime. Finite-size scaling of the superfluid stiffness clearly shows the nature of the Kosterlitz-Thouless transition. The transition temperature T(c) confirms a scaling relation T(c) proportional, rho(0)(x), with x=1.0. Some evidence of anomalous quantum behavior at low temperatures is presented.     

Monte Carlo simulation of electron swarms in nitrogen in uniformE×B fields

The motion of electrons in nitrogen in uniform E× B fields is simulated using the Monte Carlo technique for 240⩽E/N⩽600 Td (1 Td=1×10^-17 V cm²) and 0⩽B/N⩽0.45×10^-17 T cm³ . The electron-molecule collision cross sections adopted are the same cross sections as those used previously for the numerical solution of the Boltzmann equation. The swarm parameters obtained from the Monte Carlo simulation are compared with the Boltzmann solution and with the experimental data available in the literature. In relation to E×B fields, it is concluded that the Monte Carlo approach provides an independent method of substantiating the validity of the equivalent electric-field approach     

Van Vleck second moments and hydrogen diffusion in YH2.1--measurements and simulations

Proton magnetic resonance absorption spectra of yttrium dihydride (YH(2+x)), with x = 0.10, were recorded in the temperature ranges 4.2-310 K at 36.01 MHz and 150-400 K at 299.8 MHz. The evidence of proton self-diffusion follows from the changes of linewidth with temperature. The second moment of the resonance lines was determined from the experimental spectra and was compared with values calculated from the crystallographic data. The averaging effect of diffusion on the second moment was taken into account through Monte Carlo simulations of the diffusion process. The simulation was performed in a block of unit cells 5 x 5 x 5 with periodic boundary conditions. They compensated the effect of finite dimensions of the block. The calculated temperature dependence of the proton second moment values was fitted to the experimental ones. The fitting parameters were: the attempt frequency v0 and the activation energy Ea for hydrogen diffusion, assuming Arrhenius behavior of the jump frequencies vc = v0 exp(-Ea/k(B)T). In these preliminary studies, the Monte Carlo simulations were performed for tetrahedral-octahedral exchanges while direct tetrahedral-tetrahedral jumps were neglected for simplicity. Three models of hydrogen diffusion, differing in the maximum jump lengths allowed for a given model, were considered. These lengths were taken as the distances from the hydrogen attempting to jump to the first (1NN), second (2NN), and third (3NN) nearest neighbor position able to accept the jumping atom. Assuming the same attempt frequency v0 = 6.0 x 10(12)s(-1) for all three models, the activation energies giving the best fit to experimental data were 0.5, 0.54, and 0.55 eV for 1NN, 2NN, and 3NN models, respectively.     

Correlations and conductivity of interacting fermions in a disordered chain—a Monte Carlo simulation

The interplay between disorder and interaction in a one dimensional system of fermions is investigated by the use of a Monte Carlo simulation. The model considered (Hubbard Anderson Model) is a combination of the Anderson model, for noninteracting fermions in a random potential, and the extended Hubbard model, for interacting fermions in a periodic potential. To study the physics of this model, a (Quantum) Monte Carlo simulation is performed for a finite chain of 120 sites. The simulation is done for different band fillings, and several values of the interaction parameters and the strength of disorder. The low frequency behaviour of the conductivity is calculated as well as the static correlation functions for the charge density and the spin density. From the results for these quantities the competition between disorder-induced effects (Anderson localization) and interaction-induced effects (Mott transition, long range order) is studied.     

Probing the dark energy with quasar clustering

We show through Monte Carlo simulations that the Alcock-Paczyński test, as applied to quasar clustering, is a powerful tool to probe the cosmological density and equation of state parameters Omega(m0), Omega(x0), and w. By taking into account the effect of peculiar velocities upon the correlation function we obtain for the Two-Degree Field QSO Redshift Survey the predicted confidence contours for the cosmological constant (w = -1) and spatially flat (Omega(m0)+Omega(x0) = 1) cases. For w = -1, the test is especially sensitive to the difference Omega(m0)-Omega(Lambda0), thus being ideal to combine with cosmic microwave background results. For the flat case, it is competitive with future supernova and galaxy number count tests, besides being complementary to them.     

模拟锆(Zr) 的基体效应对Ni3Al-x atoms % Zr晶界内聚性的影响(英文)

利用双粒子模型研究微量元素锆(Zr)的基体效应对Ni3Al x atoms % Zr晶界内聚性的影响. 模型显示， x\[锆(Zr)的基体浓度\]从0.1增加到0.5， 在晶界Zr富集是增加的； Ni富集和Al贫乏在x=0.3时趋于最大. 模型还显示， Ni3Al x atoms % Zr晶界的内聚性在x=0.3时为最佳.     

High-energy magnon dispersion and multimagnon continuum in the two-dimensional Heisenberg antiferromagnet

We use quantum Monte Carlo simulations and numerical analytic continuation to study high-energy spin excitations in the two-dimensional S = 1/2 Heisenberg antiferromagnet at low temperature. We present results for both the transverse (x) and longitudinal (z) dynamic spin structure factors Sx,z(q,omega) at q = (pi,0) and (pi/2, pi/2). Linear spin-wave theory predicts no dispersion on the line connecting these momenta. Our calculations show that in fact the magnon energy at (pi,0) is 10% lower than at (pi/2, pi/2). We also discuss the transverse and longitudinal multimagnon continua and their relevance to neutron scattering experiments.     

Measurements of the surface ionization in multilayered specimens

Results from experimental measurements of surface ionization, Φ(0), from multilayered specimens are presented. The studied samples consisted of Cu and C layers of different thicknesses, deposited on single‐element substrates that cover the periodic system, from Be to Bi. The surface ionization was determined by the tracer method, i.e. by measuring the characteristic x‐ray intensity emitted from an ultra‐thin tracer layer deposited on the multilayer structure and, according to Castaing's definition, dividing it by the x‐ray intensity from an equivalent, self‐supporting tracer layer. The considered tracer element was Ni, and measurements were performed for Ni Kα and Ni Lα x‐rays. Experimental results are compared with Monte Carlo simulation results generated by using the general‐purpose simulation package PENELOPE with ionization cross‐sections computed from an optical‐data model. Measured data are also compared with the predictions of an empirical analytical expression for Φ(0), which was derived from systematic Monte Carlo simulations. Copyright © 2004 John Wiley & Sons, Ltd.     

Monte Carlo Simulation of Hydrogen Absorption in Palladium and Palladium-Silver Alloy

Russian Physics Journal - The paper presents Monte Carlo simulation of the hydrogen absorption in palladium and palladium-silver alloy of the composition PdxAg1–x (x = 0.7–0.9) at...     

Distribution of NMR relaxations in a random Heisenberg chain

NMR measurements of the (29)Si spin-lattice relaxation time T(1) were used to probe the spin-1/2 random Heisenberg chain compound BaCu(2)(Si(1-x)Ge(x))(2)O(7). Remarkable differences between the pure (x=0) and the fully random (x=0.5) cases are observed, indicating that randomness generates a distribution of local magnetic relaxations. This distribution, which is reflected in a stretched exponential NMR relaxation, exhibits a progressive broadening with decreasing temperature, caused by a growing inequivalence of magnetic sites. Compelling independent evidence for the influence of randomness is also obtained from magnetization data and Monte Carlo calculations. These results suggest the formation of random-singlet states in this class of materials, as previously predicted by theory.     

Stability of (3)He(2)(4)He(n) and (3)He(3)(4)He(n) L=0 clusters

We have studied the stability of mixed (3)He/(4)He clusters in L=0 states by the diffusion Monte Carlo method, employing the Tang-Toennies-Yiu He-He potential. The clusters (3)He(4)He(N) and (3)He(2)(4)He(N) are stable for N>1. The lighter atoms tend to move to the surface of the cluster. The minimum number of 4He atoms able to bind three 3He atoms in a L=0 state is nine. Two of three fermionic helium atoms stay on the surface, while the third one penetrates into the cluster.     

Diagonalizations on a correlated basis

Unsymmetrical quantum-dot systems are generally difficult to study using wave-function techniques, like quantum Monte Carlo (QMC) or exact diagonalization (ED) methods. The initial trial wave function for Monte Carlo methods is difficult to find, and the exact diagonalization method can only handle very few particles.In this article a two-dimensional semiconductor quantum dot containing a non-centered impurity ion is studied, using a new exact wave-function method. Results are analyzed and compared to density-functional-theory calculations. The computational method allows one to relax the commonly used lowest-Landau level (LLL) approximation, and it's effects are studied, e.g., on the charge and current density profiles.The method, which is a combination of QMC and ED methods, is described. It combines the scalability of Monte Carlo methods with the benefits of exact diagonalization, and allows one to accurately obtain the wave function for unsymmetrical quantum dots up to more than ten electrons. Also, excited states are accessible and are analyzed in this article.     

Kinematics of multigrid Monte Carlo

We study the kinematics of multigrid Monte Carlo algorithms by means of acceptance rates for nonlocal Metropolis update proposals. An approximation formula for acceptance rates is derived. We present a comparison of different coarse-to-fine interpolation schemes in free field theory, where the formula is exact. The predictions of the approximation formula for several interacting models are well confirmed by Monte Carlo simulations. The following rule is found: For a critical model with fundamental Hamiltonianþ(), the absence of critical slowing down can only be expected if the expansion of þ( +) in terms of the shift contains no relevant (mass) term. We also introduce a multigrid update procedure for non-abelian lattice gauge theory and study the acceptance rates for gauge groupSU(2) in four dimensions.     

The contribution of order to the phase stability of β-Cu-Zn-Al

We report the lower stability temperatures, T(β), of the body centred cubic (bcc) β phase for several alloys along the line of compositions Cu(0.76-0.5x)-Zn(x)-Al(0.24-0.5x), corresponding to a constant electron to atom ratio e/a = 1.48. The results have been obtained by means of differential scanning calorimetry measurements at various heating rates. The influence of atomic ordering on the stability of the bcc structure was evaluated using a mixed approach involving Monte Carlo simulations and the cluster variation method. It was found that the chemical short- and long-range ordering contributes to the free energy of the alloy by an amount of around 200k(B).     

Computer studies of detection efficiency of fast-neutron spectrum based on PADC using the Monte Carlo method

A FORTRAN code for simulation of SSNTD detection efficiency is designed using the Monte Carlo method in this work. The theoretical basis for the fast-neutron spectrum measurement based on the assembly of PADC¹ detector+radiator+degrader is described. The main principle of this technique is that, the recoiled protons produced in the radiator by fast-neutrons can be slowed down in the degrader and only protons of energies within a limited range can be detected by PADC (proton energy within the PADC detection energy window). Therefore with a set of different-thickness degraders one can measure the fast-neutron spectrum. Studies show that the program can give reasonable results.     

Monte Carlo methods for yrast spectroscopy

We discuss the details of the recently proposed Monte Carlo method to evaluate the exact energies of yrast levels. Energy levels are evaluated up to J = 18 with small statistical errors using the Metropolis method for the case of ¹⁶⁶Er using the pairing plus quadrupole model within one major shell. We also discuss the evaluation of the probabilities of the Hartree-Fock-Bogoliubov wave functions in the corresponding yrast eigenstates and they are found to be large. The model displays a too strong backbending behaviour not seen experimentally.Received: 29 September 2003, Published online: 24 August 2004PACS: 21.60.-n Nuclear-structure models and methods - 02.70.Ss Quantum Monte Carlo methods - 21.60.Ka Monte Carlo models - 21.10.Re Collective levels     

Extracting nuclear transparency from p,2p-A and e,e′p-A cross sections

We study nuclear-structure effects on the transparency in high-transverse-momentum (p,2p) and (e,e′p) reactions. We show that in the eikonal-distorted-wave impulse approximation (EDWIA), even when correlations are included, one can get a factorized expression for the transparency which depends only on the average nucleon density (r) and a correlation function. A technique is developed to include correlations in a Glauber calculation, where we use a Monte Carlo method for integration over the nuclear coordinates. We compare calculations of T using the EDWIA formalism, and a continuous density, with a Monte Carlo method based on discrete nucleons.